Chopawamsic Island -- a lush, secluded 13-acre dot of land situated in the Potomac River about 300 yards off the Quantico Marine base -- has been sold for $500,000 to a British-owned firm for use as a religious retreat.

"It's going to be used as a youth retreat, simply for people to get away from the demands of life -- to study, to learn and to develop," said Robert Lewis, a Woodridge real estate dealer who took part in the sale. He said the tiny crescent-shaped island would be maintained in its largely undeveloped state in an atmosphere of "Christian fellowship."

The identity of the buyer remains a mystery. Documents recording the sale have not yet been filed in Stafford County. Thomas W. Barham, a lawyer for the purchaser, declined to comment on the transaction, calling it "privileged information"

The gentleman wants to remain anonymous," said Lewis. He described the island's new owner as an American corporation controlled by one or several British stockholders.

The island's sale appears to reflect a continuing pattern in Virginia and throughout the United States of real estate purchases by foreign investors, stemming from the declining value of the American dollar, the stability of the U.S. real estate market and other factors.

Among recent foreign real estate investments in Virginia were the sale of Greenway, President John Tyler's birthplace, to West German timber magnates and the purchases of Montebello, President Zachary Taylor's reputed birthplace, by an English farmer.

Although the island is to serve as a Christian retreat, Lewis asserted, the buyer does not represent a religious cult.

"The connotation of religion has a bad taste -- people kind of think of the Hari Krishnas," Lewis said in a telephone interview. "It's not at all that type of group."

Dr. Wesley Fry, the 75-year-old retired Navy surgeon who had owned Chopawamsic from 1958 until its sale last Wednesday, expressed satisfaction both because of the purchase price and because of the new owner's plan to leave the island substantially unchanged and undeveloped.

Twenty-one years ago, Fry spent $14,000 for the then-deserted island, plus $1,500 in purchasing fees. He quickly poured considerable more than $150,000 into restoring the island's aging buildings and swampy, overgrown surroundings.

Fry said he has not yet computed his profits on the sale or decided what to do with them. "I've been doing a lot of thinking, but I haven't come to any conclusion as to how I'm going to spend the money," he said.

The asking price for the island had been $800,000. Although the property was sold for $500,000, Lewis said the return for Fry will be higher because the price will be paid with interest in long-term installments. The installment payments provide tax advantages, Lewis noted.

Chopawamsic Island now includes three restored pre-Civil War homes, a hand-dug swimming pool, five boats three tractors and a 1969 Saab automobile. It has no road or telephone. The land is filled with oak, apple, plum and cherry trees. A damp breeze blows and blue herons soar overhead in summertime.

Fry and his wife, Erma, plan to move to a 30-acre farm they recently bought near Charlottesville. A former Quantico Navel Hospital surgeon, Fry retired Aug. 31 as a member of a Veterans Administration appeals board.

"I'm a farmer now," Fry said in a telephone interview yesterday. "Just for something to do," he quickly added. "I'm not going to try to make any money out of it."

Rep. Herbert E. Harris II (D-Va.) initially expressed concern that the sale of Chopawamsic might pose a security problem. But according to an aide, Harris later consulted Marine Corps officials and concluded no security risk existed.